In recent years, various sensors are incorporated in vehicles. Examples of the sensors include a wheel-speed sensor which detects the number of rotations of wheels per second, a rudder-angle sensor which detects a degree of a turn of a steering wheel, and a yaw-rate sensor which detects a yaw speed of a vehicle. Information detected by such a sensor is supplied to a CAN (controller area network). A technique of calculating an amount of a travel distance of a vehicle by obtaining the information on the CAN has been used.
On the other hand, when the information on the CAN is used, there arise problems in that different companies employ different formats of the CAN, different types of vehicle have different information, some types of vehicle are not provided with sensors.
Accordingly, a technique of calculating a traveling amount of a vehicle using an image captured by a car-mounted camera without using the information on the CAN has been proposed. Examples of such a technique include a technique of detecting a general road sign included in an image captured by a car-mounted camera and calculating a speed of a vehicle in accordance with a size of the detected road sign. Furthermore, the examples include a technique of calculating a speed of a vehicle in accordance with a change of a position of a mark generated by mark generation means. These proposed techniques are disclosed in Japanese Laid-open Patent Publication No. 2009-205642 and Japanese Laid-open Patent Publication No. 10-123162.